1. Field of the Invention
The present invention relates to a novel plastic encapsulated type semiconductor device and a lead frame comprising Fe-Ni group alloy used for the same.
2. Description of the Prior Art
Two kinds of materials of Cu alloy and Fe-Ni group alloy are used mainly as materials for a lead frame of a plastic encapsulated type semiconductor device. The two kinds of materials have features that, while the former has good thermal conductivity and almost equal thermal expansion coefficient to a resin for encapusulation, the latter has superior oxidation resistance and almost equal thermal expansion coefficient (4.0.about.10.0.times.10.sup.-6 K.sup.-1) to Si (thermal expansion coefficient: 3.6.times.10.sup.-6 K.sup.-1) etc. Currently, accompanying with higher integration of a semiconductor chip, there is a tendency to increase heat generated from a semiconductor tip and to enlarge the size of a semiconductor chip. A lead frame made of Cu alloy is suitable for mounting of a semiconductor chip having large heat generation rate and relatively small size, but for mounting of a semiconductor chip having larger size, the lead frame has a disadvantage in that cracks develop on a device owing to a stress induced by a difference of thermal expansion of the lead frame and the semiconductor chip. Therefore, as a lead frame for mounting of a semiconductor chip of larger size; a lead frame made of Fe-Ni alloy is preferable. But, alloys by prior art such as 42% alloy (42% Ni, residual Fe) and 50% alloy (50% Ni, residual Fe) are inferior to a Cu alloy in corrosion resistance, adhesiveness with a resin, and solderability.
As to means for solving problems of the lead frame made of Fe-Ni alloy, a method to add a chemical element such as Cu to the alloy to improve the characteristics of the alloy itself is disclosed in the JP-A-63-169056 (1988), and another method to put a plating layer of Cu on surface of the lead frame to improve the characteristics of surface of the lead frame is disclosed in the JP-A-63-169056 (1988), and further a method to form a coating film of Ni, Co, Au, Ag, and Cu alloy containing Pd more than 40% is disclosed in the JP-B-63-49382 (1988).
Among the prior arts described above, either of a method to add a chemical element such as Cu etc. to a Fe-Ni alloy and a method to put a metallic layer such as Cu plating layer etc. on surface of a lead frame, are effective in improving corrosion resistance and adhesiveness with a resin of the lead frame. Nevertheless, the former method has a disadvantage of increased thermal stress in the lead frame and the semiconductor tip owing to the increment of thermal expansion coefficient by adding of a chemical element to the alloy, and the latter method has another disadvantage of lowered solderability owing to an oxidation of Cu on the surface of the lead frame in a process of packaging. And, a current trend of high integration needs increasingly higher reliability of a semiconductor package, especially a technique to prevent a crevice corrosion which will be generated at an interface of a lead frame and a resin is indispensable for improvement of reliability on humidity protection. Nevertheless, the prevention of the crevice corrosion was not considered in the prior art. Further, a coating film containing Pd more than 40% has defects when the coating film is formed by plating, and a problem has been existing that the defects accelerate corrosion.